It is known to use coaxial cable to couple television source equipment (e.g., cameras, recorders, film scanners, etc.) to associated studio equipment (e.g., mixers, special effects generators, transmitters, etc.). Where the transmission link is relatively long (say, a few hundred feet) or is subject to relatively harsh environmental conditions, it would be advantageous to provide transmission via fiber optic cable. Such cables are rugged, relatively inexpensive and provide a very wide bandwidth transmission capability. In order to avoid certain nonlinearities inherent in optical emitters and detectors, it is desirable to employ digital encoding and decoding of the video signal.
Digital transmission requires precise synchronization of the receiver (decoder) with the transmitter (encoder). This problem becomes particularly troublesome where it is desired to transmit the video signal in serial form since the data rate for the encoded signal can easily exceed 100 MBS (mega-bits per second). At such data rates a synchronization error of only a few nano-seconds can make the difference between an error free transmission and one in which the signal is grossly distorted if not completely lost.
In one form of digital transmission system the signal is encoded in pulse code modulation (PCM) form and transmitted synchronously, with the decoder being synchronized by means of a framing code that is periodically inserted in the transmitted signal. The use of framing codes advantageously increases the transmission efficiency of the system as compared with so called "start-stop" asynchronous transmissions wherein each word includes a start bit and one or two stop bits to synchronize the receiving decommutator.